Unimolecular process

Strictly unimolecular processes—sometimes also called monomoiecuiar—involve only a single particle ... 

Wardlaw D M and Marcus R A 1987 On the statistical theory of unimolecular processes Adv. Chem. Rhys. 70 231-63... 

Quack M and Tree J 1974 Specific rate constants of unimolecular processes. II. Adiabatic channel model Ber. Bunsenges. Phys. Chem. 78 240-52... 

Quack M and Troe J 1975 Complex formation in reactive and inelastic scattering statistical adiabatic channel model of unimolecular processes III Ber. Bunsenges. Phys. Chem. 79 170-83... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

The bimodal profile observed at low catalyst concentration has been explained by a combination of two light generating reactive intermediates in equihbrium with a third dark reaction intermediate which serves as a way station or delay in the chemiexcitation processes. Possible candidates for the three intermediates include those shown as "pooled intermediates". At high catalyst concentration or in imidazole-buffered aqueous-based solvent, the series of intermediates rapidly attain equihbrium and behave kineticaHy as a single kinetic entity, ie, as pooled intermediates (71). Under these latter conditions, the time—intensity profile (Fig. 2) displays the single maximum as a biexponential rise and fall of the intensity which is readily modeled as a typical irreversible, consecutive, unimolecular process ... 

Decomposition of Thiols. Thiols decompose by two principal paths (i43— i45). These are the carbon—sulfur bond homolysis and the unimolecular decomposition to alkene and hydrogen sulfide. For methanethiol, the only available route is homolysis, as in reaction 29. For ethanethiol, the favored route is formation of ethylene and hydrogen sulfide via the unimolecular process, as in reaction 30. 

Unimolecular reaction (Section 11.4) A reaction that occurs by spontaneous transformation of the starting male-rial without the intervention of other reactants. For example, the dissociation of a tertiary alkyl halide in the S l reaction is a unimolecular process. 

A number of mechanisms for thermal decomposition of persulfate in neutral aqueous solution have been proposed.232 They include unimolccular decomposition (Scheme 3.40) and various bimolecular pathways for the disappearance of persulfate involving a water molecule and concomitant formation of hydroxy radicals (Scheme 3.41). The formation of polymers with negligible hydroxy end groups is evidence that the unimolecular process dominates in neutral solution. Heterolytic pathways for persulfate decomposition can he important in acidic media. 

Single-step nucleation, (ii) above, requires the unsatisfactory assumption that the generation of a single molecule (atom, ion-pair, etc.) of product constitutes the establishment of a nucleus. (It would seem to be more realistic to regard this as the outcome of several distinct chemical steps.) The mathematical treatment expressing the probability of the occurrence of this unimolecular process is... 

Three basic types of fundamental processes are recognized unimolecular, bimolecular and termolecular. Unimolecular processes are reactions involving only one reactant molecule. Radioactive decay is an example of a unimolecular process ... 

Photolytic reactions such as the decomposition of ozone by light are also unimolecular processes ... 

The fourth and final category contains reagents that are weak nucleophiles and weak bases. These reagents include water (H2O) and alcohols (ROH), and are generally used for unimolecular processes (8 1 and El). 

In a unimolecular reaction, a molecule fragments into two pieces or rearranges to a different isomer, hi either case, a chemical bond breaks. For example, in the fragmentation of bromine molecules, breaking a ffbond gives a pair of bromine atoms Bf2 2 Br Recall that this unimolecular process is the first step of the reaction between molecular hydrogen and molecular bromine to give HBr. 

Energy profiles for two unimolecular processes (a) the unimolecular decomposition of a bromine molecule (b) the unimolecular Isomerization of ds-2-butene. 

In the search for a plausible mechanism for initiation in thermal polymerization, it is necessary to reject unimolecular processes such as the opening of the double bond to form a monomeric diradical... 

Here we give a brief account of some unimolecular processes other than isomerization. No detailed description of bimolecular processes will be offered, except to remark that (1) the knowledge gained from the unimolecular processes is often useful in interpreting the bimolecular processes and (2) in some cases, the bimolecular processes resemble normal diffusion-influenced reactions in the condensed phase. 

Similarly, the reaction of photoexcited 9,10-dicyanoanthracene (DCA) with a benzylstannane yields the contact ion pair in which the cation radical undergoes rapid mesolytic cleavage of the C—Sn bond to afford benzyl radical and tributyltin cation (which then adds to DCA- )77 (Scheme 14). When such unimolecular processes are faster than the energy-wasting back electron transfer (/cbet) within the contact ion pair, the D/A reactions occur rapidly despite unfavorable driving forces for electron transfer. 

The rate constant ke corresponds to the reciprocal of the lifetime of the excited state. Internal conversion The excited state can do other things, such as convert some of the original electronic excitation to a mixture of vibration and a different electronic state. These are also treated as unimolecular processes with associated rate constants ... 

In a gas phase, the loss of energy requires collisions, whereas in a condensed phase, it can be considered a unimolecular process.)... 

In summary, primary halides react almost wholly by a bimolecular process and tertiary halides react by a unimolecular process. Secondary halides are structurally between these two extreme structural examples, since reaction occurs by both Sn2 and SnI routes. These two mechanisms proceed in competition, and occur concurrently. 

By inspection alone, we can guess that AY will be negative for bimolecular reactions, since two components associate to form one (the TS). The value of AY is positive for unimolecular processes, such as gas-phase dissociation. 

The basic ideas presented above correspond to an analysis of a typical unimolecular process, as for instance, SN1 mechanism where the solvent may have achieved the stabilization of the di-ionic quantum state and has favored ionic dissociation as opposed to homolytic dissociation. The chemical interconversion appears here to be a quantum mechanical change of state where the solvent fluctuations would play the role of... 

Now, let us discuss the rate equations embodied in eq.(74). To do this, there is need of a statistical analysis. If the system is kept coupled to a thermostat at absolute temperature T, and assuming that w(i - >if) contains effects to all orders in perturbation theory, the rate of this unimolecular process per unit (state) reactant concentration k + is obtained after summation over the if-index is carried out with Boltzman weight factors p(if,T) ... 

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